The present invention relates generally to free-space optical communication systems and methods, and more particularly to a method of and system for transmitting information through free-space by focal plane multiplexing.
Free-space optical transmission offers an attractive alternative to conventional fiber optic transmission in military and commercial applications. Free-space optical transmission is similar to fiber optic transmission except that the light beam is transmitted through the air or space rather than through an optical fiber cable. Some of the advantages of free-space optical transmission over fiber optic transmission are low dispersion and the avoidance of having to lay optical cables. Not having to lay optical cables is particularly attractive in achieving connectivity between existing buildings in a downtown or campus environment.
A free-space optical transmission system includes an infrared laser transmitter and a receiver. The laser transmitter is positioned on one building and it is aimed at the transmitter positioned on another building. Typically, a transmitter and receiver are located at each site so that bidirectional communication can be achieved. Currently, free-space laser transmission systems can operate at data rates up to about 155 megabytes per second (Mbps) at distances over a mile. Free-space optical transmission systems operate on conventional protocols such as Ethernet, ATM, FDDI, and the like.
There is always a demand for higher data rates. While 155 Mbps is a fairly high transmission rate, there are currently Gigabit Ethernet systems. If free-space optical communication systems are to meet the demand, then their data rates must be increased by at least an order of magnitude. With the current technology, such an increase is not possible.
It is therefore an object of the present invention to increase the data rate of a free-space optical communications system.
The present invention increases the data rate of a free-space optical communications system by focal plane multiplexing. The system of the present invention includes spaced apart first and second optical elements. Each optical element has a focal plane. The system includes a plurality of optical transmitters, each of which includes a light emitting element. The light emitting elements are disposed spaced apart from each other in the focal plane of the first optical element. The system also includes a plurality of optical receivers, each of which includes a light receiving element. The light receiving elements are disposed spaced apart from each other in the focal plane of the second optical element.
Optical signals emitted by the light emitting elements are received at first optical element and combined into a single beam. The single beam is received at the second optical element. The second optical element separates the single beam into separate optical signals. The separate optical signals are focused onto the receiving elements, which are positioned on the focal plane of the second optical element.
Preferably, the optical signals are all emitted on the same optical wavelength. The first optical element effectively multiplexes the optical signals by combining them into a single beam. The second optical element demultiplexes the optical signals by focusing the separate signals on different points of its focal plane.